In a fuel injector known from German Patent No. DE 35 33 085 A1, a piezoelectric actuator is provided for actuating a valve-closure member to seal off a spray orifice, the valve-closure member being operatively connected with force-locking by way of a transfer bolt to a valve needle coupled to the valve-closure member. The piezoelectric actuator is comprised of a plurality of piezoelements arranged one behind the other in a stack. The piezoelements have a disk shape and, on both disk surfaces, have electrodes capable of receiving an electric voltage. When voltage is applied, each of the disk-shaped piezoelements, arranged one behind the other in a stack, expands in the direction of the electrical field being formed between the electrodes.
Since the expansion of the piezoelements used for the valve lift is in the direction of the electrical field, the electrodes are arranged, of necessity, on surfaces extending normal to the expansion direction. To achieve a high enough electrical field strength in each piezoelement, the layer thickness of the piezoelements must not be too great. Therefore, to achieve sufficient valve lift, it is necessary for a large number of thin piezoelements to be disposed in a stack, one behind the other, and to be pressed against one another with a suitable mechanical prestressing in the expansion direction. Since the electrodes run parallel to the contact surface where the individual piezoelements lie together contiguously, a monolithic manufacturing of the piezoelectric actuator is not possible.
Fuel injectors having a hydraulic displacement transformer are described in German Patent No. DE 43 06 073 C1 and German Patent No. DE 195 00 706 A1, where a relatively small displacement of a working piston coupled to the piezoelectric actuator is transformed into a substantially larger displacement of a reciprocating piston coupled to the valve-closure member. In this case, the piezoelectric actuators are comprised of a number of stacked piezoelements.
A drawback of the known piezoelectric actuators is that a costly manufacturing process is required to produce them. An additional drawback is that tensile stress may not be applied to the piezoelectric actuators, since this would entail the risk of the electrodes that are vapor- or sputter-deposited onto the piezoelectric crystals becoming detached. To counteract this, a mechanical compressive prestressing of the known piezo-actuators is necessary, requiring additional components.